Testing solar radiation management as a geoengineering technique

5 November 2011

Solar radiation management (SRM) is one of the geoengineering techniques proposed as a potential means of offsetting some of the anthropogenic radiative forcing of climate as a means to reduce climate change. (Earlier post.) However, the effectiveness and associated risks of SRM are uncertain.

A team of researchers from California Institute of Technology, the Carnegie Institution for Science and Harvard University has now examined the possibility of testing solar radiation management (SRM) through sub-scale deployment as a means to test models of climate response to SRM and explore risks prior to full-scale implementation. Contrary to some claims, they note, this could provide meaningful tests of the climate’s response to SRM within a decade.

The response at long time-scales would need to be extrapolated from results measured by a short-term test; this can help reduce the uncertainty associated with relatively rapid climate feedbacks, but uncertainties that only manifest at long time-scales can never be resolved by such a test...However, tests could require several decades or longer to obtain accurate response estimates, particularly to understand the response of regional hydrological fields which are critical uncertainties. Some fields, like precipitation over land, have as large a response to short period forcing as to slowly-varying changes. This implies that the ratio of the hydrological to the temperature response that results from a sustained SRM deployment will differ from that of either a short-duration test or that which has been observed to result from large volcanic eruptions.

—MacMynowski et al.

Their study is published in the RSC journal Energy & Environmental Science.

Solar radiation management is a class of theoretical concepts for manipulating the climate in order to reduce the risks of global warming caused by greenhouse gasses; however, the potential effectiveness and risks of SRM are uncertain.

Ideas for solar radiation management include increasing the amount of aerosols in the stratosphere, which could scatter incoming solar heat away from Earth’s surface, or creating low-altitude marine clouds to reflect these same rays. The size of the scale and the intricacies of the many atmospheric and climate processes make testing these ideas difficult.

The researchers turned to the Hadley Centre Coupled Model, version 3 (HadCM3L), a general circulation model of the atmosphere-ocean, for their simulations of SRM’s possible effect on the planet.

To take geoengineering methods like solar radiation management seriously, we need to build realistic models. The aim of our study was to create a framework to better understand what might be learned through testing. I do not believe that large-scale testing makes sense now.

Our goal was to examine the all-or-nothing assumption common in studies of SRM, by using climate models to find out if a limited test of SRM could be detected in the face of natural climate variability. Our results suggest that it should be possible to turn SRM on slowly—looking carefully for unexpected side-effects—before committing to full-scale use.

Using models the team was able to demonstrate that smaller-scale tests of solar radiation management could help inform decisions about larger scale deployments. Short-term tests would be particularly effective at understanding the effects of geoengineering on fast-acting climate dynamics. But testing would require several decades and, even then, would need to be extrapolated out to the centuries-long time scales relevant to studying climate change.

While it is clearly premature to consider testing solar radiation management at a scale large enough to measure the climate response, it is not premature to understand what we can learn from such tests. But we did not address other important questions such as the necessary testing technology and the social and political implications of such tests.

—Doug MacMynowski, Caltech

Some scientists have theorized that volcanic eruptions could stand in for tests, as they would cause same types of atmospheric changes as aerosols. But they wouldn’t be as effective as a sustained test.

No test can tell us everything we might want to know, but tests could tell us some things we would like to know. Tests could improve our understanding of likely consequences of intentional interference in the climate system and could also improve our knowledge about the climate’s response to the interference caused by our carbon dioxide emissions. We conducted a scientific investigation into what might be learned by testing these proposals. We are not advocating that such tests should actually be undertaken.

Comments

Full-scale SRM is already well within the means of the wealthiest individuals. The intervention required for a small-scale test is obviously open to many more, though the data-collection effort to make it worthwhile may well be more expensive than even the full-scale SRM.

Though not desirable, I am afraid that radiation geoengineering will be the only solution left 50 or 100 years from now, because honestly I don't see humankind capable of curbing its CO2 emission. So better to start collecting data as early as possible because it is not something you can improvise overnight.

Treehugger, nuclear energy can do the job of eliminating carbon emissions for electric generation. If it's done with molten-salt reactors, it looks like it can supply much industrial process heat and thermochemical processes can even produce relatively cheap biofuels from garbage. This yields the bonus of eliminating the garbage.

If we're really faced with prospects worse than losing WWII, why the heck aren't we willing to consider efforts as bold as the Manhattan project?

And unless Earth is being threatened by an exploding Sun (we're all dead anyway) or a radiating asteroid - this is pointless. If there is an asteroid - why not trigger active volcanoes which will emit billions of tons of particulates - raise albedo, lower the Earth's temp and cause acid rain in the process.

Isn't this part of a distracting agenda to stop people from thinking about the major (growing) issue USA is facing? The economic system is collapsing from endless greed and unchecked speculation and we are to worry about SRM?

There is a solution to pollution-free and CO2-free energy production. Build LFTRs. Liquid Flouride Thorium Reactors are inherently safe, use cheap thorium (literally free from rare-earth mines), do not produce any long term radio-active waste. (Here I define long term as greater than 300 years, as LFTRs do produce a tiny quantity of radio-active waste, less than 0.01% of standard Uranium reactors, that decays to safe levels in 300 years instead of 10000 years.)
See: flibe-energy.com

Besides that any LFTR-reactor would also need a neutron source (e.g. artificially bred U233) to start the reaction and also produces Pa231 with a half life of over 30,000 years: LFTRs reactors are not commercially available.
It would not be sensible to wait decades for affordable LFTR reactors to appear and not invest in commercially available and affordable efficiency measures, renewable energy options and forestation measures now. These alternative options are already available in numerous varieties now and undoubtedly can reduce greenhouse gas emissions immediately.

Pa-231 captures neutrons, then beta-decays to U-232. The half-life of U-232 is 68.9 years, and if it captures a neutron it becomes fissionable U-233.

It appears that a LFTR can be started on almost anything which can sustain a chain reaction. Enriched uranium or reclaimed (or weapons-derived) plutonium will do, and we can get hundreds of tons of the latter and tens of thousands of tons of the former.

Assuming a hypothetical LFTR-reactor was commercially available, it may be more cost effective to not waste any neutrons on Pa-231 or any decay products from the U or Pu needed to sustain the reaction and just leave this waste to future generations. After all a hypothetical LFTR-reactor would have to prevail in an increasingly competitive low-carbon energy production and energy savings market and would thus be forced to maximize its cost effectiveness.

The cost of Solar Radiation Management (SRM) should be spent on Solar Radiation Utilization, in the form of solar electricity and solar thermal energy. Fossil fuel is a finite resource and very valuable resources, and thus should be conserved as much as possible.

Soon, solar PV will be cost competitive with coal-fired electricity, and solid-state lithium battery will more cost-effective as well as much more compact than current lithium ion batteries, with much better cycling performance. The two combined technologies means very effective V2G scheme for storing excess solar and wind energy by the use of smart grid that can tell the plugged-in vehicles (PEV's) when to charge and when to discharge. H2-electrolyzers will step in when the capacity of all V2G PEV's are maxed out, to store the excess renewable energy for rainy days and for the coming winter, when energy consumption will outpace renewable energy collection.

I actually like the idea of V2Home or V2Business - where the storage functions like a UPS should the local CHP system fail. It will be nearly impossible for PV and wind (or any electromechanical source) to compete with LANR systems - especially when they move to solid state electron capture.